Question: Is it true that we only know 10% of the universe and the rest is dark matter, though if we only know 10% we don’t even know how big are universe is? – Lily

Answer: I think that you are referring to the fact that only about 17% of the matter in the Universe appears to be composed of atoms, or stuff that you and I are made of. The other 83% is “dark matter”, which is thought to composed of very heavy sub-atomic particles that only weakly interact with atoms or each other. Dark matter has yet to be detected, so its existence is only theoretical at the moment.